Apache mod_rewrite Technical Details - Apache HTTP Server Version 2.4








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Apache > HTTP Server > Documentation > Version 2.4 > RewriteApache mod_rewrite Technical Details


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This document discusses some of the technical details of mod_rewrite
and URL matching.

 API Phases
 Module Processing Order
 URL Encoding and Decoding
 Ruleset Processing
See alsoModule documentationmod_rewrite introductionRedirection and remappingRewriteRule FlagsVirtual hostsUsing RewriteMapPer-directory RewritesWhen not to use mod_rewrite


API Phases ¶

    The Apache HTTP Server handles requests in several phases. At
    each of these phases, one or more modules may be called upon to
    handle that portion of the request lifecycle. Phases include things
    like URL-to-filename translation, authentication, authorization,
    content, and logging. (This is not an exhaustive list.)

    mod_rewrite acts in two of these phases (or "hooks", as they are
    often called) to influence how URLs may be rewritten.

    First, it uses the URL-to-filename translation hook, which occurs
    after the HTTP request has been read, but before any authorization
    starts. Secondly, it uses the Fixup hook, which is after the
    authorization phases, and after per-directory configuration files
    (.htaccess files) have been read, but before the
    content handler is called.

    After a request comes in and a corresponding server or
    virtual host has been determined, the rewriting engine starts
    processing any mod_rewrite directives appearing in the
    per-server configuration. (i.e., in the main server configuration file
    and <Virtualhost>
    sections.) This happens in the URL-to-filename phase.

    A few steps later, once the final data directories have been found,
    the per-directory configuration directives (.htaccess
    files and <Directory> blocks) are applied. This
    happens in the Fixup phase.

    In each of these cases, mod_rewrite rewrites the
    REQUEST_URI either to a new URL, or to a filename.

    In per-directory context,
    rules are applied during the Fixup phase after the URL has already
    been translated to a filename. This changes what the pattern matches
    against and how substitutions are handled. See the
    Per-directory Rewrites
    document for practical details on path stripping, RewriteBase, and
    how to avoid looping.



Module Processing Order ¶

    mod_rewrite and mod_alias both
    operate during the URL-to-filename translation phase, but
    mod_rewrite runs first regardless
    of the order in which directives appear in the configuration file.
    This is determined by the hook priority each module registers, not
    by source order.

    The practical consequence: when both RewriteRule and Redirect (or RedirectMatch) are present in the
    same server or virtual-host context, the rewrite rules are
    evaluated first. If a RewriteRule matches and rewrites
    the URL-path (or returns a redirect), Redirect never sees
    the request.

    # In this configuration, the Redirect is never reached for /old
# because the RewriteRule matches first — even though
# the Redirect appears earlier in the file.
Redirect "/old" "http://example.com/new"
RewriteRule "^/old" "/other" [L]


    Per-directory context reverses the order
    In per-directory context,
    the situation is different. mod_alias directives like
    Redirect still run in the URL-to-filename translation
    phase, but mod_rewrite rules run later, in the
    Fixup phase. This means that in per-directory context,
    Redirect is evaluated before
    RewriteRule.
    

    Because of this inconsistency between contexts, mixing
    mod_rewrite and mod_alias
    directives in the same scope is a common source of confusion. The
    simplest advice: choose one module for a given task. If you need
    rewrite conditions or pattern matching, use
    RewriteRule exclusively. If a simple prefix redirect
    suffices, use Redirect and don't add rewrite rules
    that might interact with it.



URL Encoding and Decoding ¶

    Apache httpd unescapes URL-encoded characters in the request URL-path before any
    RewriteRule pattern
    matching takes place. A request for
    /my%20page/cats%3Fdogs is decoded to
    /my page/cats?dogs, and that decoded string is what the
    RewriteRule pattern matches against.

    This means you cannot write a pattern that matches the literal
    URL-encoded form. If you need to distinguish
    /horses%2Fponies from /horses/ponies, use
    %{THE_REQUEST} in a RewriteCond — it preserves the
    original request line exactly as the client sent it, before any
    decoding:

# Match only the literally-encoded %2F, not a real path separator
RewriteCond "%{THE_REQUEST}" "/horses%2F"
RewriteRule "^/horses/ponies$" "/special-handler" [L]


    After substitution, mod_rewrite re-encodes the
    resulting URL-path for output. Several flags control this behavior:

    
      [B] — re-escape
      backreferences so that special characters captured from the
      decoded URL-path are not interpreted as delimiters in the
      substitution.

      [BNP] — when [B] is
      active, encode spaces as %20 rather than
      + (appropriate for path components, not query
      strings).

      [NE] — suppress the
      default escaping of special characters in the substitution
      result, allowing literal #, ?, and
      other characters to pass through unmodified on external
      redirects.
    

    AllowEncodedSlashes
    

    By default, Apache returns 404 for any URL containing an encoded
    slash (%2F). The AllowEncodedSlashes directive controls
    this behavior:

    
      Off (default) — reject %2F with
      404.
      On — allow %2F and decode it to
      / before passing to handlers.
      NoDecode — allow %2F but keep it
      in encoded form, letting the backend application distinguish it
      from a real path separator.
    

    When using the [B] flag with
    URLs that may contain encoded slashes, you typically need
    AllowEncodedSlashes NoDecode to prevent Apache from
    rejecting the re-encoded result.

    



Ruleset Processing ¶

      Now when mod_rewrite is triggered in these two API phases, it
      reads the configured rulesets from its configuration
      structure (which itself was either created on startup for
      per-server context or during the directory walk of the Apache
      kernel for per-directory context). Then the URL rewriting
      engine is started with the contained ruleset (one or more
      rules together with their conditions). The operation of the
      URL rewriting engine itself is exactly the same for both
      configuration contexts. Only the final result processing is
      different.


      
      Figure 1:The per-request rewrite process showing
      both per-server and per-directory rule processing phases


      The order of rules in the ruleset is important because the
      rewriting engine processes them in a special (and not very
      obvious) order. The rule is this: The rewriting engine loops
      through the ruleset rule by rule (RewriteRule directives) and
      when a particular rule matches it optionally loops through
      existing corresponding conditions (RewriteCond
      directives). For historical reasons the conditions are given
      first, and so the control flow is a little bit long-winded. See
      Figure 2 for more details.

      
      Figure 2:The control flow through the rewriting ruleset

      First the URL is matched against the
      Pattern of each rule. If it fails, mod_rewrite
      immediately stops processing this rule, and continues with the
      next rule. If the Pattern matches, mod_rewrite looks
      for corresponding rule conditions (RewriteCond directives,
      appearing immediately above the RewriteRule in the configuration).
      If none are present, it substitutes the URL with a new value, which is
      constructed from the string Substitution, and goes on
      with its rule-looping. But if conditions exist, it starts an
      inner loop for processing them in the order that they are
      listed. For conditions, the logic is different: we don't match
      a pattern against the current URL. Instead we first create a
      string TestString by expanding variables,
      back-references, map lookups, etc. and then we try
      to match CondPattern against it. If the pattern
      doesn't match, the complete set of conditions and the
      corresponding rule fails. If the pattern matches, then the
      next condition is processed until no more conditions are
      available. If all conditions match, processing is continued
      with the substitution of the URL with
      Substitution.


      
      Figure 3:The flow of backreferences through a rule.
      RewriteRule Pattern is matched first; its captures ($1...$9)
      are available in all RewriteCond TestStrings. The last matched
      CondPattern's captures (%1...%9) are available in Substitution.




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